Hydrogen treating or hydrotreating is an established and important unit process in modern petroleum refining and is used with various petroleum fractions for removing heteroatom-containing contaminants such as sulfur, nitrogen, and oxygen as well as other impurities; it may also be used for saturating olefins and more unsaturated components. High pressure operation may effect partial saturation of aromatics to improve product properties or processability. Hydrotreating processes are frequently used for upgrading distillate fuels by removing sulfur and improving odor and color properties, burning qualities and, in the case of diesel fuel, cetane numbers. Hydrotreating may also be employed with catalytic cracking feeds to reduce the sulfur content of the feeds with a consequent, favorable effect on the sulfur contents of the cracking products as well as on the emissions of sulfur oxides from the cracking unit, especially the stack gases from the regenerator. The cracking properties of the feed may also be improved with more gasoline and less coke being made from hydrotreated feeds. The principal application of hydrotreating is, however, in catalytic desulfurisation since sulfur is a frequent component in petroleum-based fractions. Hydrotreating processes are described in "Petroleum Processing, Principles and Applications", R. G. Hengstebeck, McGraw- Hill, New York 1959, pp. 308-311; see also "Chemistry of Catalytic Processes", Gates et al, McGraw-Hill, New York, 1979.
Hydrotreating is generally carried out by passing the selected petroleum feedstock over a hydrotreating catalyst in the presence of hydrogen. Hydrotreating pressures typically range from about 250 to 1200 psig (1825 to 8375 kPa abs.) with higher pressures being appropriate when aromatics saturation is desired, e.g. 1500 to 2000 psig (10,445 to 13,890 kPa abs.). Temperatures, although elevated, are maintained at relatively low values since hydrogenation is exothermic with the thermodynamics of the hydrogenation reaction becoming unfavorable as temperature increases. Temperatures will typically range from about 500.degree. to 800.degree. F. (260.degree. to 425.degree. C.), and usually will be in the range of about 600.degree. to 750.degree. F. (315.degree. to 400.degree. C.). Depending upon the hydrogen consumption which will occur, hydrogen circulation rates will typically be from about 1000 to 4000 SCF/bbl (178 to 712 n.l.l..sup.-1). Hydrogen consumption is typically below 500 SCF/bbl (about 90 n.l.l..sup.-1).
The catalysts used in hydrotreating generally comprises a metal hydrogenation component on an amorphous, porous support material. The hydrogenation component comprises a transitional metal usually selected from Groups VIA and VIIIA of the Periodic Table (IUPAC Table). The preferred metals for this purpose are the base metals, especially nickel, cobalt, molybdenum and tungsten, of which cobalt and molybdenum are especially preferred, particularly in combination with one another. The support material may contribute a limited degree of acidic functionality to the catalyst which is desirable in order to effect removal of the organic heteroatoms by promoting the opening of heterocyclic rings containing these atoms at low to moderate degrees of desulfurisation. Alumina, silica-alumina and silica are the normal support materials for this purpose.
Zeolites have not usually been employed as the support, either on their own or combined with an amorphous matrix such as alumina, because of their greater activity, with the concommitant increase in conversion and reduction in yield. However, if deep desulfurisation e.g. 95% or more, is required, a greater degree of acidic functionality may be required in order to effect ring opening of heterocyclic aromatic feed components prior to conversion of the organic heteroatoms to inorganic sulfur and nitrogen. Zeolite containing catalysts of this type are reported to be effective for deep hydrodesulfurization, as described by Vrinat et al: "Catalytic Hydrodesulfurization of Dibenzothiophene Over Y-type Zeolites", Catalysis by Zeolites, B. Imelik et al (Ed.) Elsevier 1980, p. 219. Because the heteroatoms are usually contained in aromatic ring systems, a catalyst which is aromatic selective, such as zeolite Y, would be the catalyst of choice for providing the required degree of acidic functionality, as reported by Vrinat.
The problems encountered with hydrodesulfurization are especially notable with highly aromatic feeds such as the cycle oils obtained from catalytic cracking processes. These cycle oils generally comprise bicyclic and tricyclic aromatic ring compounds from which the paraffinic side chains have been removed during the catalytic cracking process. Depending upon the severity of the cracking process, short alkyl side chains may be present on these aromatics but they are relatively refractory to further catalytic cracking, even in the presence of a conventional cracking catalyst such as zeolite Y.
Zeolites, especially zeolites X and Y, have been used for some time in hydrocracking catalysts where their relatively greater activity is an asset and it has also been found that they provide excellent resistance to aging, especially with the more highly siliceous forms such as zeolite USY. Their effectiveness as a component of hydrocracking catalysts is predicated upon their aromatic selective character: they attack the aromatic components of the feed in preference to the paraffinic components. This aromatic-selective behavior of the large pore size zeolites X and Y is not, however, matched by zeolite beta, another zeolite which possesses many characteristics consistent with those of a large pore size zeolite, as noted by Higgins et al. Zeolites 8,446-452 (1988) and Treacy et al. Nature 332,249-251 (1988). Zeolite beta, by contrast, is paraffin-selective, attacking the paraffinic components of a feed in preference to the aromatics so that when a feed containing both aromatic and paraffinic comonents is processed over zeolite beta, the paraffinic components are removed first with the aromatic components tending to remain until higher conversion is attained. This paraffin-selective behavior of zeolite beta is described in U.S. Pat. No. 4,419,220 (Lapierre et al.) and EP94327. The marked selectivity of zeolite beta for paraffins in preference to aromatics implies that it would not be expected to function effectively as an acidic component in a hydrodesulfurization catalyst with highly aromatic feeds such as the cycle oils obtained from catalytic cracking and aromatic extracts from solvent extraction processes. We have found, however, that zeolite beta is unexpectedly effective as a component of hydrodesulfurization catalysts for such feeds and is able to effect a significant degree of desulfurization while maintaining relatively low levels of boiling range conversion.